[Baz]

I don't know why you couldn't just make the base square instead of round. You're just talking about the base. A round fountain could still sit just fine on a square base.

And it wouldn't affect the landscaping one bit. Just proceed with a "rounded" design around the square base.

Otherwise the suggestions upstream in the thread would help you get the job done, should you wish to go forward with a round concrete base.

Personally, I've been using pavers or washed shell for most stuff like that here. A lot less hassle and less permanent.